1. Field of the Invention
The present invention relates to a process of manufacturing a semiconductor device, and more particularly, to a method of inspecting a circular mask used to form a fine pattern by exposure in a process of manufacturing a semiconductor device.
2. Description of the Related Art
As the precision of exposure processes increases in the manufacture of semiconductor devices, small mask errors can cause serious wafer defects. Therefore, several thousand or more points of a mask are required to be inspected to achieve a sufficient level of reliability in patterns formed in the mask.
However, since the processing time of current mask inspection equipment, such as a scanning electron microscope (SEM), is slow, only several tens of points of a mask can be inspected. Moreover, optical differences of a mask (e.g., material variations, height variations, or pattern slopes of the mask) that affect formation of a pattern in a wafer cannot be measured using the SEM. In addition, although the entire surface of a mask can be inspected using conventional optical inspection equipment, the optical effects of the mask cannot be precisely measured using the conventional optical inspection equipment since the conventional optical inspection equipment have a low resolution and use a light source having a wavelength different from the wavelength used in an exposure process.
FIG. 1 is a graph illustrating relationships between critical dimension (CD) sensitivity and sampling rate in conventional mask inspection equipment. In FIG. 1, the x-axis represents a sampling rate (a ratio of inspected area to the whole area), and the y-axis represents sensitivity for a CD error of a mask. When the sensitivity decreases, CD errors can be more accurately detected.
Referring to FIG. 1, a SEM can precisely detect CD errors only for a very small area of a mask. Similarly, an aerial image measurement system (AIMS) can accurately detect CD errors of a mask only for a small area of a mask; however, an AIMS is less accurate than a SEM. Since a SEM or an AIMS can inspect only a small area of a mask, not all CD errors of the mask can be detected using a SEM or AIMS. Furthermore, the inspection speed of a SEM or AIMS is very slow. In addition, the optical effects of a mask cannot be measured using a SEM as described above.
Meanwhile, although an aerial image inspection apparatus (AERA) is similar to an AIMS, the AERA can inspect the entire surface of a mask at a high speed using a single pulse. However, as illustrated in FIG. 1, since the CD sensitivity of the AERA is not good (i.e., having a higher sensitivity), a mask pattern cannot be precisely measured using the AERA.
Referring to FIG. 1, a star marks a point where the sampling rate is 100% and the CD sensitivity is 0%, which are ideal parameters for a mask inspection method or apparatus.